This description relates to spectrally adjustable filtering.
Devices that spectrally filter and/or detect light can be used in optical communication systems, such as wavelength division multiplexed (WDM) systems, for adjustable filtering and attenuation of spectral components (or “wavelengths” or “channels”) that are multiplexed together in the WDM spectrum. Such devices can be used for optical detection and monitoring of spectral components in the WDM spectrum. Typically, each spectral component has a nominal wavelength corresponding to the center frequency of a channel. In some systems, each spectral component has a spectral shape with a central lobe that has a narrow enough linewidth such that adjacent spectral components, whose center frequencies are separated by a given channel spacing, do not overlap significantly. In some systems, spectral components comprise respective sub-bands of wavelengths over a continuous spectrum that does not necessarily have any separation between adjacent spectral components. It is understood that the term “spectral component” as used herein should be construed broadly, and encompasses these various examples and other examples as understood in the art.
Wavelength filtering devices are typically classified as fixed or adjustable. Fixed devices filter a predetermined spectral band while adjustable filters allow reconfiguration of the wavelengths being filtered. Adjustable filtering devices provide the ability to dynamically update system configuration to optimize parameters such as transmission performance and bandwidth utilization. Adjustable filters also enable system operators to provision (turn up) new wavelengths and services.
Optical spectrum monitoring devices can be used within a WDM system to monitor system performance by measuring one or more optical signal attributes including wavelength, optical power, optical signal-to-noise ratio (OSNR), spectral shape, bit-rate, and polarization state, among others. The information provided by optical spectrum monitors may be utilized for system health reporting, fault location and analysis, feedback control and system optimization, and for other purposes.
Some optical component designs use optical elements in imaging configurations to enable and adjust functionality and performance. Some spectral filters utilize elements in imaging configurations. That is, they utilize elements to image beams or spectral components originating at one point in space to an intersection at a different point in space within the plane of spectral dispersion. These spectral filters place functional elements at the convergence points of the imaging configurations. Moreover, the functional elements are characteristically placed at distances from the imaging elements, e.g. lenses, that are equal to or greater than the focal lengths of the imaging elements.
A disadvantage of utilizing imaging configurations is that the optical path lengths of such configurations can be exceedingly long as the placement of the functional components are dictated by the focal lengths of the imaging elements (lenses). It is generally not possible to simply reduce the focal length without compromising spectral performance or cost or both. In particular, lens aberration effects (and associated spectral performance degradation) combined with the associated cost premiums of aspheric lenses that can provide acceptable performance and reliability, typically limit the applicability of imaging configurations. Accordingly, for many current WDM systems, which place a significant premium on size, performance, and cost, devices that employ imaging configurations may be at a disadvantage and may not even be realizable for some applications and requirements.